This invention relates to methods and apparatus for synchronizing multiple electric motor driven generators and, more particularly, to methods and apparatus for connecting multiple alternating current motor driven generators to a common output bus, where the type of generator is synchronous, the type of motor is either synchronous or induction, and the motors are driven from either a common source bus, or from separate but phase-related source busses.
A wide variety of alternating current motor driven generators have been designed over the years to, for example, generate AC power having a frequency which is different from the frequency of the utility supplied AC power. A typical application for such motor driven generators is to provide four-hundred hertz power to a load when only a sixty hertz power source is available. Another application is to provide uninterrupted power to a load in the event of a short term outage of the utility supplied power. In this instance, the mechanical inertia of the rotating portions of the motor driven generator is relied upon to provide power during the outage.
In the use of motor driven generators, it is often necessary to connect more than one motor driven generator to a common output bus without interrupting power to the load. For example, it may be necessary to provide more power to the load than is available from a single generator, or it may be necessary to transfer the load from one generator to another without interrupting power to the load. When a second synchronous generator is connected to an output bus which is already carrying power from a first synchronous generator it is necessary that the output voltages of the two generators be approximately equal, and it is also necessary to make the connection at a time when the voltage from the second generator is in phase with the output bus voltage. Connecting an out-of-phase generator to the output bus can cause series disturbances to the load, as well as cause damage to the rotating elements.
The task of phase synchronizing a synchronous generator is further complicated if that generator is driven by a synchronous motor. This type of motor poses the additional constraint that it must be phase synchronized to the source voltage for proper operation. In those applications where the source frequency and the generator output frequency are not the same, phase synchronization becomes an even more difficult task.
One way in which phase synchronization of motor driven generators has been accomplished in the past is to use a mechanical alignment system. In this system, the second motor driven generator is brought up to speed, and the motor stator is then physically rotated with respect to the generator stator. This rotation has the effect of shifting the phase of the output voltage of that generator. The stator rotation is continued unitl the second generator voltage is in phase with the output bus voltage. A major problem with this arrangement is the need for complicated and expensive mechanical apparatus such as bearings, gearboxes, control motors and control logic. Generally, this type of mechanical alignment system is limited to vertical motor driven generator configurations in which the motor is mounted above the generator. Horizontal configurations create difficult alignment and support problems with respect to providing a rotatable motor stator.
Accordingly, it is an object of the present invention to provide new and improved methods and apparatus for synchronizing multiple motor driven generators.
It is another object of the present invention to provide methods and apparatus for synchronizing multiple motor driven generators which do not require rotatable stators.
It is yet another object of the present invention to provide methods and apparatus for synchronizing multiple motor driven generators using a microprocessor control system.